Estimation and mapping of ablation volume

ABSTRACT

Tissue ablation systems and methods are provided, wherein a cardiac catheter incorporates a pressure detector for sensing a mechanical force against the distal tip when engaging an ablation site. Responsively to the pressure detector, a controller computes an ablation volume according to relationships between the contact pressure against the site, the power output of an ablator, and the energy application time. A monitor displays a map of the heart which includes a visual indication of the computed ablation volume. The monitor may dynamically display the progress of the ablation by varying the visual indication.

This application is a divisional application of U.S. patent applicationSer. No. 12/646,197, filed on Dec. 23, 2009, now U.S. Pat. No.8,926,604, the entire contents of which are hereby incorporated byreference herein.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to relates generally to minimally invasivetreatment of organs inside the body. More particularly, this inventionrelates to methods and devices for prediction and assessment of ablationtreatments applied to cardiac tissue.

Description of the Related Art

Intracardiac radio-frequency (RF) ablation is a well known method fortreating cardiac arrhythmias. Typically, a catheter having an electrodeat its distal tip is inserted through the patient's vascular system intoa chamber of the heart. The electrode is brought into contact with asite (or sites) on the endocardium, and RF energy is applied through thecatheter to the electrode in order to ablate the heart tissue at thesite. It is important to ensure proper contact between the electrode andthe endocardium during ablation in order to achieve the desiredtherapeutic effect without excessive damage to the tissue.

Various techniques have been suggested for verifying electrode contactwith the tissue. For example, U.S. Pat. No. 6,695,808, whose disclosureis incorporated herein by reference, describes apparatus for treating aselected patient tissue or organ region. A probe has a contact surfacethat may be urged against the region, thereby creating contact pressure.A pressure transducer measures the contact pressure. This arrangement issaid to meet the needs of procedures in which a medical instrument mustbe placed in firm but not excessive contact with an anatomical surface,by providing information to the user of the instrument that isindicative of the existence and magnitude of the contact force.

As another example, U.S. Pat. No. 6,241,724, whose disclosure isincorporated herein by reference, describes methods for creating lesionsin body tissue using segmented electrode assemblies. In one embodiment,an electrode assembly on a catheter carries pressure transducers, whichsense contact with tissue and convey signals to a pressure contactmodule. The module identifies the electrode elements that are associatedwith the pressure transducer signals and directs an energy generator toconvey RF energy to these elements, and not to other elements that arein contact only with blood.

A further example is presented in U.S. Pat. No. 6,915,149, whosedisclosure is incorporated herein by reference. This patent describes amethod for mapping a heart using a catheter having a tip electrode formeasuring local electrical activity. In order to avoid artifacts thatmay arise from poor tip contact with the tissue, the contact pressurebetween the tip and the tissue is measured using a pressure sensor toensure stable contact.

U.S. Patent Application Publication 2007/0100332, whose disclosure isincorporated herein by reference, describes systems and methods forassessing electrode-tissue contact for tissue ablation. Anelectro-mechanical sensor within the catheter shaft generates electricalsignals corresponding to the amount of movement of the electrode withina distal portion of the catheter shaft. An output device receives theelectrical signals for assessing a level of contact between theelectrode and a tissue.

Visualization of ablation lesions in real time is important in enablingthe physician to ensure that each point along the treatment path hasbeen sufficiently ablated to interrupt conduction, while avoiding thedangers of excessive ablation. U.S. Pat. No. 7,306,593, issued to Keidaret al., whose disclosure is herein incorporated by reference, describesa method for ablating tissue in an organ by contacting a probe insidethe body with the tissue to be ablated, and measuring one or more localparameters at the position using the probe prior to ablating the tissue.A map of the organ is displayed, showing, based on the one or more localparameters, a predicted extent of ablation of the tissue to be achievedfor a given dosage of energy applied at the position using the probe.The given dosage of energy is applied to ablate the tissue using theprobe, and an actual extent of the ablation at the position is measuredusing the probe subsequent to ablating the tissue. The measured actualextent of the ablation is displayed on the map for comparison with thepredicted extent.

SUMMARY OF THE INVENTION

It has been found experimentally that the volume of heart tissue ablatedwhen RF energy is applied by a catheter electrode in contact with thetissue at a given point is roughly proportional to the RF power (P) androughly proportional to the mechanical force (F) between the catheterand the tissue. Thus, the P*F product gives a good indication of therate of ablation of the tissue and may be used in real-time mapping ofthe volume of tissue ablated.

An embodiment of the invention provides a method of ablation, which iscarried out by inserting a probe into a body of a living subject, urgingthe probe into contact with a tissue in the body, determining amechanical force that is exerted by the probe against the tissue, andapplying a specified dosage of energy to the tissue for ablationthereof, wherein at least one of the application time of the dosage andthe power level depend on the mechanical force.

An aspect of the method is performed prior to applying the specifieddosage of energy by reporting an indication of an expected ablationvolume at the power level, the application time and the mechanicalforce.

A further aspect of the method includes displaying a visual indicationof the ablation volume, and responsively to the visual indicationcontrolling the ablation volume by varying at least one of the powerlevel, the mechanical force and the application time.

Another aspect of the method includes calculating a rate of ablation asa function of the power level and the mechanical force, and controllingthe rate of ablation by varying at least one of the power level and themechanical force.

Still another aspect of the method includes monitoring tissuetemperature of the tissue and controlling the rate of ablation isperformed responsively to the temperature.

Other embodiments of the invention provide apparatus for carrying outthe above-described method.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the detailed description of the invention, by way of example, whichis to be read in conjunction with the following drawings, wherein likeelements are given like reference numerals, and wherein:

FIG. 1 is a pictorial illustration of a system for performing ablativeprocedures on a heart of a living subject, which is constructed andoperative in accordance with an embodiment of the invention;

FIG. 2 is a cutaway view of the distal end of a catheter used in thesystem shown in FIG. 1, in accordance with an embodiment of the presentinvention;

FIG. 3 is a pictorial view of the distal end of the catheter shown inFIG. 2 in contact with endocardial tissue, in accordance with anembodiment of the invention;

FIG. 4 is a composite map of the heart, illustrating aspects of acardiac ablation procedure in accordance with an embodiment of theinvention;

FIG. 5 is a flow chart of a method of estimation and mapping tissueablation volume, in accordance with a disclosed embodiment of theinvention; and

FIG. 6 is a cutaway view of a catheter used in the system shown in FIG.1, which is constructed and operative in accordance with an alternateembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the various principles ofthe present invention. It will be apparent to one skilled in the art,however, that not all these details are necessarily always needed forpracticing the present invention. In this instance, well-known circuits,control logic, and the details of computer program instructions forconventional algorithms and processes have not been shown in detail inorder not to obscure the general concepts unnecessarily.

Turning now to the drawings, reference is initially made to FIG. 1,which is a pictorial illustration of a system 10 for performing ablativeprocedures on a heart 12 of a living subject, which is constructed andoperative in accordance with a disclosed embodiment of the invention.The system comprises a catheter 14, which is percutaneously inserted byan operator 16 through the patient's vascular system into a chamber orvascular structure of the heart. The operator 16, who is typically aphysician, brings the catheter's distal tip 18 into contact with theheart wall at an ablation target site. Electrical activation maps maythen be prepared, according to the methods disclosed in U.S. Pat. Nos.6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No.6,892,091, whose disclosures are herein incorporated by reference.Although the embodiment described with respect to FIG. 1 is concernedprimarily with cardiac ablation, the principles of the invention may beapplied, mutatis mutandis, to other catheters and probes and to bodytissues other than the heart.

Areas determined to be abnormal by evaluation of the electricalactivation maps can be ablated by application of thermal energy, e.g.,by passage of radiofrequency electrical current through wires in thecatheter to one or more electrodes at the distal tip 18, which apply theradiofrequency energy to the myocardium. The energy is absorbed in thetissue, heating it to a point (typically about 50° C.) at which itpermanently loses its electrical excitability. When successful, thisprocedure creates non-conducting lesions in the cardiac tissue, whichdisrupt the abnormal electrical pathway causing the arrhythmia.Alternatively, other known methods of applying ablative energy can beused, e.g., ultrasound energy, as disclosed in U.S. Patent ApplicationPublication No. 2004/0102769, whose disclosure is herein incorporated byreference. The principles of the invention can be applied to differentheart chambers, and to mapping in sinus rhythm, and when many differentcardiac arrhythmias are present.

The catheter 14 typically comprises a handle 20, having suitablecontrols on the handle to enable the operator 16 to steer, position andorient the distal end of the catheter as desired for the ablation. Toaid the operator 16, the distal portion of the catheter 14 containsposition sensors (not shown) that provide signals to a positioningprocessor 22, located in a console 24. The console 24 typically containsan ablation power generator 25. The catheter 14 may be adapted toconduct ablative energy to the heart using any known ablation technique,e.g., radiofrequency energy, ultrasound energy, and laser energy. Suchmethods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733,6,997,924, and 7,156,816, which are herein incorporated by reference.

The positioning processor 22 is an element of a positioning sub-systemof the system 10 that measures location and orientation coordinates ofthe catheter 14.

In one embodiment, the positioning sub-system comprises a magneticposition tracking arrangement that determines the position andorientation of the catheter 14 by generating magnetic fields in apredefined working volume its vicinity and sensing these fields at thecatheter. The magnetic position tracking arrangement typically comprisesa set of external radiators, such as field generating coils 28, whichare located in fixed, known positions external to the patient. The fieldgenerating coils 28 are driven by field generators (not shown), whichare typically located in the console 24, and generate fields, typicallyelectromagnetic fields, in the vicinity of the heart 12.

In an alternative embodiment, a radiator in the catheter 14, such as acoil, generates electromagnetic fields, which are received by sensors(not shown) outside the patient's body.

Some position tracking techniques that may be used for this purpose aredescribed, for example, in the above-noted U.S. Pat. No. 6,690,963, andin commonly assigned U.S. Pat. Nos. 6,618,612 and 6,332,089, and U.S.Patent Application Publications 2004/0147920, and 2004/0068178, whosedisclosures are all incorporated herein by reference. Although thepositioning sub-system shown in FIG. 1 uses magnetic fields, the methodsdescribed below may be implemented using any other suitable positioningsystem, such as systems based on electromagnetic fields, acoustic orultrasonic measurements. A suitable commercial positioning sub-system isthe CARTO XP EP Navigation and Ablation System, available from BiosenseWebster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765.

As noted above, the catheter 14 is coupled to the console 24, whichenables the operator 16 to observe and regulate the functions of thecatheter 14. Console 24 includes a processor, preferably a computer withappropriate signal processing circuits. The processor is coupled todrive a monitor 30. The signal processing circuits typically receive,amplify, filter and digitize signals from the catheter 14, includingsignals generated by the above-noted sensors and a plurality of sensingelectrodes (not shown) located distally in the catheter 14. Thedigitized signals are received and used by the console 24 to compute theposition and orientation of the catheter 14 and to analyze theelectrical signals from the electrodes. The information derived fromthis analysis may be used to generate an electrophysiological map of atleast a portion of the heart 12 or structures such as the pulmonaryvenous ostia, for diagnostic purposes such as locating an arrhythmogenicarea in the heart or to facilitate therapeutic ablation.

Typically, the system 10 includes other elements, which are not shown inthe figures for the sake of simplicity. For example, the system 10 mayinclude an electrocardiogram (ECG) monitor, coupled to receive signalsfrom one or more body surface electrodes, so as to provide an ECGsynchronization signal to the console 24. As mentioned above, the system10 typically also includes a reference position sensor, either on anexternally-applied reference patch attached to the exterior of thesubject's body, or on an internally-placed catheter, which is insertedinto the heart 12 maintained in a fixed position relative to the heart12. By comparing the position of the catheter 14 to that of thereference catheter, the coordinates of catheter 14 are determinedrelative to the heart 12, irrespective of heart motion. Alternatively,any other suitable method may be used to compensate for heart motion.Nevertheless, the positioning sub-system cannot guarantee that anenergy-conveying component of the catheter 14 is in actual contact withthe tissue to be ablated.

Reference is now made to FIG. 2, which is a cutaway view of distal end33 of catheter 14 (FIG. 1), showing details of the structure of thecatheter in accordance with an embodiment of the present invention. Thecatheter shown in FIG. 2 includes a pressure transducer, which is morefully disclosed in commonly assigned U.S. Patent Application PublicationNo. 2009/0093806, which is herein incorporated by reference. Other knowntypes of pressure transducers can be substituted for the pressuretransducer described with reference to FIG. 2.

Catheter 14 comprises a flexible insertion tube 54, with a distalsection 72 connected to the remainder of the insertion tube 54 at ajoint 56. The insertion tube is covered by a flexible, insulatingmaterial 60, such as Celcon™ or Teflon™. The area of joint 56 iscovered, as well, by a flexible, insulating material, which may be thesame as material 60 or may be specially adapted to permit unimpededbending and compression of the joint, (This material is cut away in FIG.2 in order to expose the internal structure of the catheter.) Distal tip18 may be covered, at least in part, by an electrode 50, which istypically made of a metallic material, such as a platinum/iridium alloy.Alternatively, other suitable materials may be used, as will be apparentto those skilled in the art. The distal section 72 is typicallyrelatively rigid, by comparison with a proximal section 74.

The distal section 72 is connected to the proximal section 74 by aresilient member 58. In FIG. 2, the resilient member 58 has the form ofa coil spring, but other types of resilient components may alternativelybe used for this purpose. For example, resilient member 58 may comprisea polymer, such as silicone, polyurethane, or other plastics, with thedesired flexibility and strength characteristics. Resilient member 58permits a limited range of relative movement between distal section 72and the proximal section 74 in response to forces exerted on the distalsection 72 or directly against the distal tip 18. Such forces areencountered when the distal tip is pressed against the endocardiumduring an ablation procedure. The desired pressure for good electricalcontact between the distal tip and the endocardium during ablation is onthe order of 20-30 grams. The spring serving as the resilient member 58in this embodiment may be configured, for example, to permit axialdisplacement (i.e., lateral movement along the axis of catheter 14) ofthe distal end 33 by about 1-2 mm and angular deflection of the distalsection 72 with respect to the proximal section 74 by up to about 30degrees in response to a desired pressure.

As noted above, distal section 72 contains a magnetic position sensor62. Position sensor 62 may comprise one or more miniature coils, andtypically comprises multiple coils oriented along different axes.Alternatively, position sensor 62 may comprise another type of magneticsensor, such as a Hall effect or magnetoresistive sensor, for example.The magnetic fields created by the field generating coils 28 (FIG. 1)cause the position sensor 62 to generate electrical signals, withamplitudes that are indicative of the position and orientation ofposition sensor 62 relative to the fixed frame of reference of fieldgenerating coils 28. Positioning processor 22 (FIG. 1) receives thesesignals via wires (not shown in the figures) running through catheter14, and processes the signals in order to derive the location andorientation coordinates of distal tip 18 in this fixed frame ofreference, as described in the patents and patent applications citedabove. Some of the position sensing and mapping features of the catheter14 are implemented in the NOGA-STAR catheter and the and CARTO™ systems,marketed by Biosense Webster, Inc.

In addition, catheter 14 contains a miniature magnetic field generator64 near the distal tip 18, which is driven by a current conveyed throughcatheter 14 from console 24 (FIG. 1). The current is generated so as tocreate magnetic fields that are distinguishable in time and/or frequencyfrom the fields of field generating coils 28 (FIG. 1). For example, thecurrent supplied to field generator 64 may be generated at a selectedfrequency in the range between about 16 kHz and 25 kHz, while fieldgenerating coils 28 are driven at different frequencies. Additionally oralternatively, the operation of field generating coils 28 and fieldgenerator 64 may be time-multiplexed.

The magnetic field created by field generator 64 causes one or morecoils in position sensor 62 to generate electrical signals at the drivefrequency of field generator 64. The amplitudes of these signals varydepending upon the location and orientation of distal tip 18 relative toproximal section 74. Positioning processor 22 (FIG. 1) processes thesesignals in order to determine the axial displacement and the magnitudeof the angular deflection of the distal tip 18 relative to the proximalsection 74. Position sensor 62 may determine six position andorientation coordinates (X, Y, Z directions and pitch, yaw and rollorientations) of the distal end and distal tip of catheter 14. For thispurpose, at least two sensing coils are typically required in theposition sensor. In the present embodiment, three sensing coils 76 areused, in order to improve the accuracy and reliability of the positionmeasurement. Alternatively, if only a single sensing coil is used,system 10 may be able to determine only five position and orientationcoordinates (X, Y, Z directions and pitch and yaw orientations). As thereadings of displacement and deflection should be accurate to within afew tenths of a millimeter and about one degree, respectively, it isdesirable to include three coils 76 in position sensor 62, preferablymutually orthogonal, as shown in FIG. 2.

As the position of the position sensor 62 with reference to some fixedframe of reference (not shown) can be determined, it is possible tocompute the relative movement of the distal tip 18 relative to theproximal section 74. This gives a measure of the deformation and angulardeviation of resilient member 58. Generally speaking, the deformation isproportional to the mechanical force that is exerted on the resilientmember 58, which is roughly equal to the force that is exerted on thedistal tip 18 by the heart tissue with which the distal tip 18 is incontact. Thus, the combination of field generator 64 with positionsensor 62 serves as a pressure sensing system for determining theapproximate pressure exerted by the endocardial tissue on the distal tip18 of the catheter 14 (or equivalently, the pressure exerted byelectrode 50 against the endocardial tissue).

Reference is now made to FIG. 3, which is a pictorial view of the distalend 33 of the catheter 14 in contact with endocardium 70 of the heart12, in accordance with an embodiment of the invention. Pressure exertedby the distal tip 18 against the endocardium 70 deforms the endocardialtissue slightly, so that the electrode 50 contacts the tissue over arelatively large area. Since the electrode 50 engages the endocardium 70at an angle 82, rather than head-on, distal section 72 bends at joint 56forming a bend angle 84 relative to the insertion tube of the catheter.The bend facilitates optimal contact between the electrode and theendocardial tissue.

Reverting to FIG. 2, positioning processor 22 (FIG. 1) receives andprocesses the signals generated by position sensor 62 in response to themagnetic field of generator 64, in order to derive an indication of thepressure exerted by distal tip 18 on endocardium 70 (FIG. 3). As notedearlier, for good ablation, pressure of about 20-30 grams is desirable.Lower pressure means that there may be inadequate contact betweenelectrode 50 and the endocardial tissue. As a result, much or all of thethermal energy may be carried away by the blood inside the heart, andthe tissue will be ablated inadequately or not at all. Higher pressuremeans that the electrode is pressing too hard against the endocardialtissue. Excessive pressure of this sort may cause cavitation in thetissue, leading to extensive tissue damage and possibly even perforationof the heart wall.

It is possible to determine the coordinates of the position sensor 62with respect to some fixed frame of reference. In embodiments in whichthe field generator 64 has at least two coils it is also possible todetermine the directional orientations of the axes of the positionsensor 62 with respect to one another, and thereby compute the bendangle 84 (FIG. 3).

By virtue of the combined sensing of displacement and deflection, thispressure sensing system reads the pressure correctly regardless ofwhether the electrode engages the endocardium head-on or at an angle.The pressure reading is insensitive to temperature variations and freeof drift, unlike piezoelectric sensors, for example.

The magnitudes of the displacement and deflection may be combined byvector addition to give a total magnitude of the movement of distal tip18 relative to the proximal section 74. When there are three coils, thesystem can determine the position of the distal section 72 and thedistal tip 18 with six degrees of freedom. Force vectors 78, 80 can thenbe computed, the vector 80 representing the magnitude of the componentthat is normal to the wall of the heart 12. The relationships betweenforce and deflection may be pre-calibrated for each catheter and acalibration table constructed and used subsequently in forcemeasurements.

Referring again to FIG. 1, console 24 outputs an indication of thepressure measured to the operator 16, and may issue an alarm if thepressure is too low or too high. Optionally, ablation power generator 25may be interlocked, so as to supply power to electrode 50 (FIG. 2) onlywhen the pressure against the endocardium 70 (FIG. 3) is in the desiredrange. Alternatively or additionally, the pressure indication may beused in closed-loop control of an automated mechanism for maneuveringand operating catheter 14, as described more fully in the above notedU.S. Patent Application Publication No. 2004/0102769, in order to ensurethat the mechanism causes the distal section 72 to engage theendocardium 70 (FIG. 3) in the proper location and with the appropriatecontact pressure.

While RF power is discussed with respect to the methods and systemsherein, in embodiments of the system 10 (FIG. 1), other forms of energymay be delivered to the tissue, i.e., laser and microwave techniques,and high intensity focused ultrasound energy, as described in commonlyassigned U.S. Patent Application Publication No. 2006/0287648, which isherein incorporated by reference.

The product P*F gives a good indication of the rate of ablation of thetissue, where P represents RF power and F represents the magnitude ofthe force vector exerted by the catheter against the endocardial surfaceof the heart. The operator may increase or decrease either or both ofthe component parameters, P and F in order to control the ablation rate.The total volume V of tissue ablated, up to a maximum dictated by tissuecharacteristics and safety considerations, is roughly proportional tothe productV≈k(P*F*T)  (1),wherein T is the time duration of RF power application, and k is aproportionality constant.

Reference is now made to FIG. 4, which is a composite map 86 of theheart illustrating aspects of a cardiac ablation procedure in accordancewith a disclosed embodiment of the invention. The procedure may beactual or simulated, for purposes of predicting the necessary force tobe applied by a cardiac catheter 88 in an operating position within achamber of heart 90. Arrows 92, 94 represent two different forcevectors, the length of the arrows corresponding to the magnitudes of theforces being. A dosage of energy, e.g., RF ablation current is to beapplied at a predetermined power level for a time sufficient to producean ablation lesion. Predicted small and large circular ablation zones96, 98 correspond to the short and long arrows 92, 94, respectively.

Additionally or alternatively, when the force being applied and the RFpower are known, the size of the ablation zone can be predicted anddynamically displayed. The completeness of the ablation can becalculated as time varies, and progress displayed during the procedureas by changing the visual characteristics of the ablation zones 96, 98.The ablation volume grows over time in proportion to the product P*F.

Similarly, by fixing the desired size of the ablation zone, the requiredforce can be computed at a given RF power and application time or for agiven total energy dosage at different combinations of application timeand RF power.

Using the map 86, a simple, clear measure of estimated ablation volumeis provided to the operator, which can be measured easily and accuratelyin near real-time.

Reference is now made to FIG. 5, which is a flow chart of a method ofestimation and mapping of tissue ablation volume, in accordance with adisclosed embodiment of the invention. The method requires adetermination of mechanical force developed by contact between a probeand the tissue site to be ablated. The method can be performed by thesystem 10 (FIG. 1) using catheter 14. However, other methods that arecapable of measuring the pressure can be applied, for exampleimpedance-based measurements, such as disclosed in commonly assignedU.S. Patent Application Publication No. 2007/0060832, whose disclosureis herein incorporated by reference. Alternatively, suitable optical orultrasound techniques may be used to determine the mechanical force.

The process begins at initial step 100. The heart is catheterizedconventionally and the catheter navigated to a desired location at whichtissue ablation is required.

Next, at step 102, the cardiac catheter is brought into contact with theendocardial surface, generally at an angle of incidence other thanperpendicular as shown in FIG. 3.

Next, at step 104, The mechanical force or a desired force vectorapplied to the endocardium by the catheter is determined. The deflectionangle, e.g., angle 82 (FIG. 3) may be determined automatically, usingthe information provided by location sensors in the catheter.

Next, at step 106, ablation power, e.g., RF power, is determined for thecurrent medical procedure.

Then, at step 108, an estimated ablation time is tentatively chosen,which establishes the energy dosage to be applied. Alternatively, steps106, 108 can be modified to set the ablation time, and estimate powerlevels, respectively. The operator may be assisted at this step in thata controller may report an indication of an expected ablation volume atthe energy dosage and the mechanical force.

Next, at step 110 the size of the lesion to be created by ablating iscomputed, according to the conditions established in step 104 and step108.

Control now proceeds to decision step 112, where it is determined if thecurrent lesion size is acceptable. If the determination at decision step112 is affirmative, then control proceeds to final step 114, wherepower, typically RF power, is applied, and ablation is performed. Duringthe ablation the currently ablated tissue volume is dynamicallydisplayed as shown in FIG. 4 until the computed ablation volume has beenachieved. The operator may vary the power to control the time ofapplication. Additionally or alternatively the operator may adjust theposition of the catheter to vary the mechanical force applied to theendocardial tissues.

If the determination at decision step 112 is negative, then controlreturns to step 108, where the ablation time is re-estimated.

Typically, the size of the lesion to be created by ablation is known. Insuch cases, the loop defined by step 108, step 110 and decision step 112can be iterated automatically until an acceptable size has beendetermined.

Alternatively the lesion size may be computed directly at optional step116 using the relationship of Equation 1, and then perform ablation atfinal step 114. In this case, step 108, step 110 and decision step 112can be omitted.

In alternate embodiments of the method, proposed power and proposedapplication time data can be received as input and ablation volumescomputed at different mechanical forces of contact with the tissue.

ALTERNATE EMBODIMENT

Reference is now made to FIG. 6, which is a cutaway view of distal end33 of catheter 14 (FIG. 1), which is constructed and operative inaccordance with a disclosed embodiment of the invention. This embodimentis similar to FIG. 2, except now the distal section 72 includes aconventional temperature sensor 118 that is capable of detectingabnormal rise in temperature of the tissues at the operating site. Bydisplaying the output of the temperature sensor 118 on the monitor 30(FIG. 1) or providing a suitable audible alert, the rate of ablation maybe controlled by responsively to the temperature of the tissues in orderto prevent charring or dangerous temperature elevation outside thecomputed ablation volume.

Equation 1 can be modified to account for the temperature such that onlyactual ablation time, rather than total elapsed time is taken intoconsideration. Ablation time can be defined to run only when contactforce exceeding a predetermined force threshold is ascertained and thetemperature exceeds a predetermined temperature threshold.Alternatively, ablation time can be defined to run only when contactforce exceeding a predetermined force threshold is ascertained or thetemperature exceeds a predetermined temperature threshold.

Equation 1 may be modified in several ways to account for ablation time.The following examples are practical approximations, in which variousfirst and second order corrections are not shown for clarity ofpresentation. The threshold values given below are suitable:V≈k*(P*F*T(F>F _(threshold)))  (2)V≈k*(P*F*T(F>F _(threshold) ,t>t _(threshold)))  (3)V≈k*(P*F*T(t>t _(threshold))  (4)wherein F_(threshold)=5 gr, and t_(threshold)=47° C. The ablation poweris applied only during time intervals when the conditions shown are met.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

What is claimed is:
 1. A method of ablation, comprising the steps of:inserting a probe into a body of a living subject; urging the probe intocontact with a tissue in the body; determining a mechanical force thatis exerted by the probe against the tissue; calculating a rate ofablation as a function of a power level and the mechanical force;applying a specified dosage of energy for an application time and at thepower level to the tissue for ablation thereof, wherein at least one ofthe application time of the dosage and the power level depend on themechanical force; and controlling the rate of ablation by varying atleast one of the power level and the mechanical force.
 2. The methodaccording to claim 1, further comprising the step of prior to applyingthe specified dosage of energy, reporting an indication of an expectedablation volume at the power level, the application time and themechanical force.
 3. The method according to claim 1, wherein the probehas a distal end and an axis, and urging the probe comprises forming anangular deflection of the distal end with respect to the axis.
 4. Themethod according to claim 1, further comprising the steps of calculatingan ablation volume of the tissue as a function of the power level, themechanical force and the application time.
 5. The method according toclaim 4, further comprising the steps of: displaying a visual indicationof the ablation volume; and responsively to the visual indicationcontrolling the ablation volume by varying at least one of the powerlevel, the mechanical force and the application time.
 6. The methodaccording to claim 1, further comprising displaying a visual indicationof the rate of ablation, wherein controlling the rate of ablation isperformed responsively to the visual indication.
 7. The method accordingto claim 1, further comprising monitoring a temperature of the tissue,wherein controlling the rate of ablation is performed responsively tothe temperature.